1. Field of the Invention
The present invention relates to a wire bonding method which uses a bonding apparatus for assembling semiconductor integrated circuits, etc.
2. Prior Art
In bonding apparatuses, bonding action is performed by (a) first actuating a bonding arm provided with a bonding tool through which a wire is passed, so that the bonding tool is lowered at a rapid speed toward the bonding surface of a sample, and then (b) changing the speed at a speed-change point which is sufficiently close to the bonding surface, so that the bonding tool presses against the bonding surface while being lowered at a slower speed.
Conventionally, the speed-change point in the lowering motion of the bonding tool is positioned at a preset position.
Meanwhile, methods for detecting the bonding surface are disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) No. 55-244 (hereinafter called "Conventional Method 1"), in Japanese Patent Application Publication (Kokoku) No. 64-9730 (called "Conventional Method 2"), and in Japanese Patent Application Publication (Kokoku) No. 64-811 ("Conventional Method 3").
As shown in FIG. 4, in Conventional Method 1, a lead frame 2 to which a pellet 1 adheres is guided and moved onto a sample-carrying table 3. A bonding arm 6 which holds a bonding tool 5 through which a wire 4 is passed is provided so as to pivot on an elevator table 7. This elevator table 7 is guided and supported so that it can move vertically on a base 9 which is installed on an X-Y table 8. The elevator table 7 is moved up and down by a feeder means which comprises a feed screw 10 supported on the base 9, a Z-axis motor which rotates the feed screw 10 and an encoder 12 which is installed on the rear end of the Z-axis motor 11.
The wire 4 is alternately bonded to the bonding surfaces of the pellet 1 and lead frame 2 by a combination of a vertical movement of the bonding tool 5 caused by the Z-axis motor 11 and an X-Y movement of the bonding tool 5 effected by the X-Y table 8.
Contacts 14 and 15 are respectively installed on the elevator table 7 and on a holder 13 which is shaft-supported together with the bonding arm 6 (the contacts 14 and 15 face each other). When the bonding tool 5 is lowered so that it is pressed against the bonding surface, the bonding arm 6 pivots by a very small angle so that the contacts 14 and 15 are separated from each other. As a result, the timing of the bonding tool contacting with the bonding surface is known.
In the above methods, the position of the speed-change point is set at a height which allows sufficient tolerance with respect to fluctuations in the height of the bonding surface caused by variations between samples and inclination of the sample surfaces. As a result, the time required for the bonding tool 5 to actually contact with the bonding surface (when the tool bonding 5 is lowered) can be not longer than necessary, resulting in that the efficiency of the bonding operation is decreased.
On the other hand, if the speed-change point is set too low, the bonding conditions (bonding load, tool lowering speed, etc.) will become unstable in cases where the bonding surface is higher than the expected position, etc. Such instability causes defective units.